1. Field of the Invention
The present invention relates to a projection exposure method and apparatus and, more particularly, to a projection exposure method and apparatus, which are used in the manufacture of a semiconductor element, a liquid crystal element, or the like in a photolithography process, and project a mask pattern onto a photosensitive substrate via a projection optical system.
2. Related Background Art
Conventionally, along with the miniaturization of patterns of semiconductor integrated circuits, a projection exposure apparatus must correct a change in imaging characteristics (e.g., a magnification, focal length, and the like) caused upon absorption of exposure light by a projection optical system. For example, as disclosed in U.S. Pat. No. 4,624,551 or U.S. Pat. No. 4,801,977, a projection exposure apparatus comprises a mechanism for correcting a variation of the optical characteristics of a projection optical system caused by illumination light absorption by detecting the amount of light incident on the projection optical system. This correction mechanism will be briefly described below. For example, a shutter OPEN signal during an exposure operation is received, the change amount of the optical characteristics is always calculated in accordance with a model, and the imaging characteristics are corrected to a predetermined value based on the change amount of the imaging characteristics by, e.g., moving at least one optical element constituting the projection optical system, or sealing a space sandwiched between two optical elements, and controlling the pressure in the sealed space.
In recent years, along with the miniaturization of patterns of semiconductor integrated circuits and the like, in a projection exposure apparatus, a change in imaging characteristics (e.g., the magnification, focal length, and the like of a projection optical system) caused by absorption of exposure illumination light by a reticle must be corrected. In view of this need, as disclosed in, e.g., Japanese Laid-Open Patent Application No. 4-192317, a mechanism for correcting the imaging state by detecting the thermal deformation amount of a reticle caused by absorption of illumination light has been proposed.
This mechanism will be briefly described below. The thermal deformation amount of a reticle caused by absorption of illumination light is numerically calculated on the basis of the heat absorbance of a reticle, the density distribution of a pattern, and the like, or is obtained by directly measuring the measurement mark position on the reticle. Based on the obtained thermal deformation amount, a change in imaging state of the projection optical system is predicted by an actual measurement or calculation. Then, correction is performed to make the imaging state constant or to minimize the influence of the variation of the imaging state on the basis of the prediction result using a correction means of the imaging state.
In the above-mentioned techniques, a problem associated with absorption of exposure light by the projection optical system is solved for the present. However, since exposure light rays are also transmitted through a mask, the mask thermally deforms due to absorption of exposure light, and the imaging characteristics change due to the thermal deformation of the mask.
In particular, since a pattern is drawn on the mask using, e.g., chromium, a chromium portion considerably absorbs heat unlike a glass portion having a high transmittance. Furthermore, in recent years, in order to prevent flare of an optical system, a technique for lowering the reflectance of a chromium portion on the mask tends to be adopted. With this technique, heat absorption by the chromium portion is promoted.
Upon heat absorption by the chromium portion, the temperature of the glass portion of the mask also increases, and the entire mark may thermally expand. According to an actual measurement, an increase in temperature of the mask is about 5.degree. C. under the worst condition. Since the material of the mask is generally quartz glass, and its expansion coefficient is 0.4 ppm/.degree. C., the temperature rise of about 5.degree. C. causes a deviation of 0.02 .mu.m with respect to a 10 mm interval, and causes a distortion error or magnification error on the image plane.
The chromium pattern on the mask is not always uniformly distributed on the entire surface of the mask, and its distribution state is locally uneven. In this case, the temperature of the mask locally increases, and an anisotropic distortion may be generated. Also, when only a portion of the mask is exposed using a light-shielding band (variable field stop), an anisotropic distortion may be similarly generated. Such a distortion of the mask causes an anisotropic distortion in an image to be projected. In this case, the distortion cannot be sufficiently removed by correcting only magnification components.
As described above, when the mask thermally deforms, the imaging characteristics deviate from normal characteristics depending on the type of mask used in the conventional technique. More specifically, when another type of mask is used, since it generally has different thermal deformation characteristics, the imaging characteristics cannot be sufficiently corrected. When exposure is performed by sequentially exchanging masks, since the conventional technique does not consider thermal deformation characteristics of each of these masks, large errors are generated.
As a countermeasure against this problem, for example, a mask may be cooled to a predetermined temperature. However, since the glass surface temperature and the temperature of the chromium portion of the mask cannot be set constant, it is impossible to cool the entire mask to have a uniform thermal distribution. Since cooling is a phenomenon accompanying heat conduction and has poor response characteristics, it cannot quickly follow OPEN and CLOSE requests of a shutter.
Also, it is difficult to accurately measure the ratio of pattern existence on a reticle under the influence of the pattern shape on the pattern surface of the reticle, the material (chromium or the like) forming the pattern, and the like.
In a reticle with a small pattern (with the low ratio of pattern existence), since the reticle undergoes almost no expansion by heat absorption of illumination light, the considerable portion of the overall error is accounted for by error components caused by correction itself, and final imaging characteristics may be worsened as compared to those before correction.
These points which have not posed serious problems in terms of accuracy in the conventional apparatus are expected to be of extreme importance for projection patterns, which are miniaturized more and more in recent years and will be miniaturized more and more in future.